Application Information

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4880 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. This shutdown feature turns the am-

plifier off when a logic high is placed on the shutdown pin.

The trigger point between a logic low and logic high level is

typically half supply. It is best to switch between ground and

the supply to provide maximum device performance. By

switching the shutdown pin to V

rent draw will be minimized in idle mode. While the device

will be disabled with shutdown pin voltages less than V

the idle current may be greater than the typical value of 0.7

µA. In either case, the shutdown pin should be tied to a defi-

nite voltage because leaving the pin floating may result in an

unwanted shutdown condition.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry which pro-

vides a quick, smooth transition into shutdown. Another solu-

tion is to use a single-pole, single-throw switch in conjunction

with an external pull-up resistor. When the switch is closed,

the shutdown pin is connected to ground and enables the

amplifier. If the switch is open, then the external pull-up re-

sistor will disable the LM4880. This scheme guarantees that

the shutdown pin will not float which will prevent unwanted

state changes.

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a

successful design.

Equation (1) states the maximum power

dissipation point for a single-ended amplifier operating at a

given supply voltage and driving a specified output load.

P

(1)

Since the LM4880 has two operational amplifiers in one

package, the maximum internal power dissipation point is

twice that of the number which results from

Equation (1).

Even with the large internal power dissipation, the LM4880

does not require heat sinking over a large range of ambient

temperatures. From

Equation (1), assuming a 5V power sup-

ply and an 8

Ω load, the maximum power dissipation point is

158 mW per amplifier. Thus the maximum package dissipa-

tion point is 317 mW. The maximum power dissipation point

obtained must not be greater than the power dissipation that

results from

Equation (2):

P

(2)

For the LM4880 surface mount package,

θ

and T

T

find the maximum internal power dissipation supported by

the IC packaging. If the result of

Equation (1) is greater than

that of

Equation (2), then either the supply voltage must be

decreased, the load impedance increased, or the ambient

temperature reduced. For the typical application of a 5V

power supply, with an 8

Ω load, the maximum ambient tem-

perature possible without violating the maximum junction

temperature is approximately 96˚C provided that device op-

eration is around the maximum power dissipation point.

Power dissipation is a function of output power and thus, if

typical operation is not around the maximum power dissipa-

tion point, the ambient temperature may be increased ac-

cordingly. Refer to the Typical Performance Characteris-

tics curves for power dissipation information for lower output

powers.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is criti-

cal for low noise performance and high power supply rejec-

tion. The capacitor location on both the bypass and power

supply pins should be as close to the device as possible. As

displayed in the Typical Performance Characteristics sec-

tion, the effect of a larger half supply bypass capacitor is im-

proved low frequency PSRR due to increased half-supply

stability. Typical applications employ a 5V regulator with

10 µF and a 0.1 µF bypass capacitors which aid in supply

stability, but do not eliminate the need for bypassing the sup-

ply nodes of the LM4880. The selection of bypass capaci-

tors, especially C

quency PSRR, click and pop performance as explained in

the section, Proper Selection of External Components

section, system cost, and size constraints.

AUTOMATIC SHUTDOWN CIRCUIT

As shown in

Figure 2, the LM4880 can be set up to automati-

cally shutdown when a load is not connected. This circuit is

based upon a single control pin common in many head-

phone jacks. This control pin forms a normally closed switch

with one of the output pins. The output of this circuit (the volt-

age on pin 5 of the LM4880) has two states based on the

state of the switch. When the switch is open, signifying that

headphones are inserted, the LM4880 should be enabled.

When the switch is closed, the LM4880 should be off to mini-

mize power consumption.

Typical Application

Frequency Response

DS012343-32

Typical Application

Frequency Response

DS012343-33

Power Derating Curve

DS012343-34

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